This thesis is an investigation of what antimicrobials are used in Ontario swine nurseries, why they are used and whether antimicrobial use was associated with performance. Firstly, a study was conducted using existing data from a survey of veterinarians. The veterinarians recorded visits to nursery farms to investigate disease problems and treatment recommendations were recorded. A second study involved a survey of Ontario nurseries to examine treatment records and antimicrobial use. A subset of 20 farms with records for feed consumption and growth rate were further studied to determine antimicrobial consumption from medicated feed. It was concluded that growth rate was not associated with amount of antimicrobials consumed during the nursery period. As an industry the quantity of antimicrobials used could be greatly reduced, with likely very little loss in performance, if producers were to only medicate feed for the first half of the nursery phase.